1. Field of the Invention
The present invention relates to a method for producing feedstocks of high quality lube base oil from unconverted oil and, more particularly, to an improvement in efficiency along with a method for continuous production of high quality lube base oil from unconverted oil produced by a fuels hydrocracker in recycle mode.
2. Description of the Prior Art
In general, a fuels hydrocracker is a process for converting vacuum gas oil (VGO) produced from a vacuum distillation unit (V1) into fuel grade hydrocarbons such as diesel fuel (as shown in FIG. 1). The VGO feed contains a large amount of impurities such as sulfur, nitrogen, oxygen, metals and other materials that are not only harmful to the catalyst system but also undesirable in the products. Such impurities are removed in a hydrotreating reaction unit (R1). The resulting hydrotreated VGO undergoes hydrocracking in a main reactor (R2) to convert a major part of it into light hydrocarbons. The reactor effluents are first separated into a hydrogen-rich gas and hydrocarbon liquid. The hydrogen rich gas is then recycled back to the two reactors (R1 and R2) while the hydrocarbon liquid is fractionated into several different grades of petroleum products through a series of fractionators (Fs). Since it is essentially impossible to accomplish 100% conversion in the reaction, a portion of the VGO feed not converted to diesel and lighter products ends up as a final fractionator bottom stream.
In fact, fuels hydrocrackers are normally designed such that the per-pass conversion (conversion achieved by a single passage through a hydrocracking reactor) is approximately 60%. The unconverted oil (UCO) is then either sent to storage as a semi-final product (this type of operation is called "once-through mode") or recycled back to the main reactor (R2) for further cracking to increase the overall conversion (this type of operation is called "recycle mode").
The UCO, because it is a mixture of highly saturated hydrocarbons, has many desirable characteristics such as a high viscosity index, which is one of the most important properties of a lube base oil. Table 1 shows typical properties for VGO and UCO for overall conversion of 85% and per-pass conversion of 60%.
TABLE 1 ______________________________________ The Properties of the VGO and the UCO Properties VGO UCO ______________________________________ API Gravity 22 38 Distillation* .degree.C. IBP** / 5% 260/340 350/370 10% / 20% 372/396 385/398 30% / 40% 415/434 410/422 50% / 60% 445/460 435/446 70% / 80% 475/492 458/474 90% / 95% 516/538 496/515 FBP*** / %recovery 547/98.5 536/99.0 Hydrogen, wt% 12.0 15.0 Nitrogen, wppm 800 4.0 Sulfur, wt% 3.0 0.0009 Aniline point .degree.C. 78 118 Pour Point .degree.C. 33 38 Viscosity, cst @ 40.degree. C. 49.9 19.3 @ 60.degree. C. 19.4 10.7 @ 100.degree. C. 6.35 4.4 Viscosity Index 64 143 Saturation Degree of 31 98 Hydrocarbon, wt% ______________________________________ *ASTM D1160, @ 760 mmHg **Initial Boiling Point ***Final Boiling Point
From an economic standpoint, it is more advantageous to utilize the UCO for high quality lube base oil after further processing such as dewaxing and stabilization than to use UCO as a fuel oil blending stock or to recycle it to the hydrocracking reactor. Some refineries are known to be producing lube base oil with a very high viscosity index using the UCO generated from a fuels hydrocracker. For example, a refinery produces VHVI (Very High Viscosity Index) lube base oil at their lube base oil plant utilizing the UCO from their fuels hydrocracker with once-through mode. The hydrocracker plant is located far away from the lube base oil plant.
However, the above conventional method for manufacturing lube base oil from the UCO in that plant has several problems. The UCO generated from the fuels hydrocracker is fed to the lube base oil plant. In that process, several existing units are being utilized including a vacuum distillation unit, a solvent extraction unit, a solvent dewaxing unit and so on in a "blocked mode" which is quite cumbersome with rather low operation efficiency.
The above-mentioned plant, especially is inefficient because the existing vacuum distillation unit was originally designed for processing atmospheric residue (AR). It is even necessary to blend the UCO with heavier stocks such as vacuum residue (VR) before feeding it to the existing vacuum distillation unit. For a better understanding of the background of the present invention, the description for a typical fuels hydrocracker in recycle mode is given below. Refer to the enclosed FIG. 1.
Atmospheric residue (AR) is fed into a first vacuum distillation unit (V1) to produce a vacuum gas oil (VGO). The VGO is then hydrotreated in a first reactor (R1) to remove impurities such as sulfur, nitrogen, oxygen and metals. The resulting treated VGO is then hydrocracked to yield a variety of hydrocarbon products in a second reactor (R2). These hydrocarbons are separated in a series of fractionators (Fs) to produce various light oil products and diesel oil.
However, not all of the cracked hydrocarbons are converted into diesel and lighter products. A substantial portion of the hydrocarbons remain unconverted. Most of such unconverted oil is sent back to the second reaction unit (R2) for further conversion. With high-endpoint vacuum gas oil feedstocks, however, heavy refractory hydrocarbons and condensed polynuclear aromatic compounds could gradually accumulate in the fuels hydrocracker's internal recycle oil stream. An excessive concentration of these compounds can cause a rapid decline in catalyst performance and a degradation in product selectivity. In order to avoid such operational instability, a small bleed stream of unconverted oil becomes necessary to purge these compounds from the system and to maintain a suitable level of reaction activity. For that purpose, in general, the fuels hydrocracker in recycle mode recycles a small portion of the product fractionator bottoms back to the feed vacuum column (V1).
The purpose of such a recirculation scheme is to reject a portion of the refractory components and polynuclear aromatics to the vacuum residue. Such a scheme also minimizes the quantity of unconverted oil that must be purged from the product fractionator bottoms. The typical recirculation rate to the feed vacuum column is 15 to 25 liquid volume % of the total unconverted oil.
In addition, the unconverted oil from the fuels hydrocracker with high conversion has an average viscosity ranging from 4.0 to 4.5 cst at 100.degree. C., which is too low to make 150 Neutral lube base oil. The 150 Neutral lube base oil is one of the grades with high demand and has viscosities ranging from 5.5 to 6.0 cst at 100.degree. C. Consequently, a considerable amount of the unconverted oil at most of the existing refineries as stated above is not being utilized for lube oil production, and wasted typically in the form of fuel oil.